Expression pattern ofBrachyuryin the molluscPatella vulgatasuggests a conserved role in the establishment of the AP axis in Bilateria

We report the characterisation of a Brachyury ortholog (PvuBra) in the marine gastropod Patella vulgata. In this mollusc, the embryo displays an equal cleavage pattern until the 32-cell stage. There, an inductive event takes place that sets up the bilateral symmetry, by specifying one of the four initially equipotent vegetal macromeres as the posterior pole of all subsequent morphogenesis. This macromere, usually designated as 3D, will subsequently act as an organiser. We show that 3D expresses PvuBra as soon as its fate is determined. As reported for another mollusc (J. D. Lambert and L. M. Nagy (2001) Development 128, 45-56), we found that 3D determination and activity also involve the activation of the MAP kinase ERK, and we further show that PvuBra expression in 3D requires ERK activity. PvuBra expression then rapidly spreads to neighbouring cells that cleave in a bilateral fashion and whose progeny will constitute the posterior edge of the blastopore during gastrulation, suggesting a role for PvuBra in regulating cell movements and cleavage morphology in Patella. Until the completion of gastrulation, PvuBra expression is maintained at the posterior pole, and along the developing anterior-posterior axis. Comparing this expression pattern with what is known in other Bilateria, we advocate that Brachyury might have a conserved role in the regulation of anterior-posterior patterning among Bilateria, through the maintenance of a posterior growth zone, suggesting that a teloblastic mode of axis formation might be ancestral to the Bilateria.